The present invention is a process for preparing alkoxysilanes by the contact of silicon with a solution of hydrogen fluoride in a liquid primary or secondary alcohol. The hydrogen fluoride activates the silicon causing the formation of alkoxysilanes to occur more rapidly and can provide selectivity for the production of dialkoxysilanes and trialkoxysilanes. The process may be run in the presence of copper or a copper compound as a catalyst. The process is advantageously run using atomized silicon.
Silicon is known to react with methanol in the presence of a strong base to form tetramethoxysilane in near quantative yield. For example, Japanese Patent J6-3041-482-A, Feb. 22, 1988, describes a process where silicon is reacted with methanol in the presence of sodium methoxide to form tetramethoxysilane. The process was run at 140.degree. C., for one hour, with the reported yield of tetramethoxysilane being about 95%. based on silicon added to the process. This process does not produce significant quantities of dialkoxysilanes or trialkoxysilanes.
Silicon is also known to react with alcohols in the presence of copper and copper compounds to form alkoxysilanes. Rochow, U.S. Pat. No. 2,473,260, issued Jun. 14, 1949, reports the reaction of silicon with methanol, in the presence of copper or a copper compound, to form a mixture containing 40-50 percent tetramethoxysilane, and polysiloxanes. Typical reaction temperatures for the process were in the range of about 250.degree. C. to 300.degree. C. with contact times of 1.75 to 5.5 hours.
de Wit, U.S. Pat. No. 3,071,700 issued Jan. 8, 1963, describes a process where silicon is reacted with an alcohol in the presence of a copper catalyst, to form dialkoxysilanes and trialkoxysilanes. The process was conducted by passing alcohol vapor through a fluidized-bed of silicon along with an inert carrier gas. The preferred process temperature was reported to be 200.degree. C. to 350.degree. C., with reaction times of 5 to 8 hours. Near quantative conversion of methanol is reported. A similar process is reported in Japanese Patent J6-3027-493-A, May 2, 1988.
Bonitz, U.S. Pat. No. 3,505,379, issued Apr. 7, 1970, describes a process for production of alkoxysilanes where finely divided silicon is reacted in the liquid phase by contact with alcohols or phenols at 20.degree. C. to 300.degree. C. The process is run in the presence of an organic catalyst or metal or metal compound soluble in the liquid phase. The production of mono-, di-, tri-, and tetra-methoxysilanes is reported.
Rochow, U.S. Pat. No. 3,641,077, issued Feb. 8, 1972, describes a process where particulate silicon is suspended in a silicone oil containing a catalyst, such as copper, and an alcohol is introduced beneath the surface of the silicone oil. The process is reported to produce di-, tri-, and tetra-alkoxysilanes. The process was run at a temperature of about 250.degree. C. t 300.degree. C., with contact times of 4-5 hours. Muraoka, U.S. Pat. No. 3,775,457, issued Nov. 27, 1973, reports a similar process where the particulate silicon is suspended in a synthetic oil containing multicyclic aromatic hydrocarbons.
Medicino, U.S. Pat. No. 4,727,173, issued Mar. 31, 1987, reports a process preferential for the production of trialkoxysilane. The process consists of reacting silicon with an alcohol in the presence of a catalytic effective concentration of copper (II) hydroxide.
Medicino, U.S. Pat. No. 4,762,939, issued Aug. 9, 1988, describes a process for producing trialkoxysilane and tetraalkoxysilane mixtures by the direct reaction of silicon with an alcohol in the presence of a copper catalyst. The process is conducted in the presence of mixed solvents, where at least one solvent is inert and at least one other solvent promotes the reaction between trialkoxysilanes and alcohol.
Forwald, EP Application 0-372-918. Pub. Jun. 13, 1990, describes a process for the production of an atomized silicon alloy that is reported to have an improved reaction rate, in comparison to ground silicon, when reacted with an alkylchloride or arylchloride.
The objective of the present invention is to provide a process that can provide significant yields of dialkoxysilanes and trialkoxysilanes at faster rates and shorter reaction times than those previously reported in the literature. This objective is achieved by using a solution of hydrogen fluoride in alcohol to activate the silicon and make the silicon more reactive with the alcohol. The reaction of the activated silicon with the alcohol is further facilitated by the addition of copper or copper compounds to the process as a catalyst. In addition, particulate silicon prepared by atomization has been found to be considerably more reactive in the process than ground particulate silicon.
The alkoxysilanes obtained by the present process contain reactive alkoxy groups and in some cases hydrogen. These reactive constituents make the alkoxysilanes useful as intermediates for preparing various organic silicon compounds or hydride functional polysiloxanes.